Embodiments described herein relate to an automatic pipetting apparatus provided with leak detection and a method of detecting a leak. In particular, the embodiments relate to an automatic pipetting apparatus which detects a leak in the piping system, particularly a leak caused at a joint between a nozzle base and a nozzle tip fitted to the nozzle base, by monitoring changes in internal pressure of the piping system and a method of detecting such a leak.
There are known pipetting apparatuses for pipetting fluid samples into a plurality of containers. These pipetting apparatuses are used to pipette fluid samples, such as blood extracted from a human body, into a plurality of containers. Such pipetting apparatuses are mainly employed in sample inspection sections in hospitals or the like for carrying out pretreatment pipetting operations such as a rough distribution of fluid samples to be inspected (e.g. blood plasma or blood serum) or biotechnological analysis of fluid samples, or pre-inspection pipetting operations such as radioimmunoassay (RIA) or enzyme immunoassay (EIA). Most of these pipetting apparatuses are configured into automatic pipetting apparatuses as shown in FIG. 1, in which a disposable type nozzle tip (disposable tip) as shown in FIG. 2 is used.
In the pipetting apparatus shown in FIG. 1, there is provided with a nozzle 32 for aspirating and dispensing fluid samples. The nozzle 32 is communicated with a pump through an air hose for supplying an air pressure. As clearly illustrated in FIG. 2, the nozzle 32 is constructed from a nozzle base 35 and a disposable tip 36 fitted to the nozzle base 35. In more details, at the top portion of the disposable tip 36, there is formed an opening into which the tip portion of the nozzle base 35 is tightly fitted. On the other hand, at the lower end of the disposable tip 36, there is formed a small opening 36a through which fluid samples are aspirated and dispensed. Such a disposable tip 36 is formed of a plastic material or the like, and the nozzle base 35 is formed of a metal or the like.
Now, in order for a prescribed volume of sample to be accurately pipetted, it is necessary that a proper volume of sample has been aspirated during the aspiration step. However, if there exist any air leaks (hereinafter referred to simply as "leaks or leak") in the sample aspirating passage running from the nozzle 32 to the pump 44 via the air hose, namely if there exist any leaks anywhere within the piping system, such as at the joint portions or junctions between the parts of the piping system, it will not be possible to aspirate a proper volume of sample.
Further, if such a leak exists in the piping system, there will be another problem in that any sample thus aspirated will experience a decrease in volume due to leakage of fluid sample caused by the leak in the piping system. As a result, the volume of sample to be dispensed will be insufficient, thus leading to a deterioration in the pipetting accuracy.
For these reasons stated above, in current pipetting apparatuses, a piping system thereof is examined to determine whether or not a leak exists before pipetting operations are begun. In general, the following method of examining the piping system is carried out before pipetting operations are begun. Namely, after a fluid sample has been aspirated, the nozzle is raised to place the lower end portion of the nozzle at a position above the surface of the fluid sample source. Next, with the nozzle being held motionless for a prescribed length of time, the lower end portion of the nozzle is monitored visually to determine whether or not any of the fluid sample drips from the lower end of the disposable tip. Then, a determination of the presence or absence of a leak in the piping system is made based on whether or not fluid sample drips from the disposable tip.
However, because the above method of detecting leaks is carried out only once before pipetting operations are begun, it is not possible to detect leaks occurring at the joint portion between the nozzle base 35 and the disposable tip 36 in the course of the pipetting operations. Such leaks occurring during the pipetting operations are, for example, resulted from the following cases.
Namely, there is dispersion in product quality of manufactured disposable tips because they are manufactured by molding. Therefore, if a normal quality disposable tip has been used at the time when the above described leak detecting method is carried out, no leak occurs at the joint portion between the nozzle base 35 and the disposable tip 36. However, if a poor quality disposable tip may have been used during actual pipetting operations which are subsequently carried out after the leak detecting method, such a poor disposable tip gives rise to leaks which cause the problems as described above.
Further, if foreign matter (hairs, etc.) is present at the contact portion between the nozzle base 35 and the disposable tip 36, leaks are also likely to arise at the joint portion during the pipetting operation.
Furthermore, because the disposable tip 36 is frequently replaced, the nozzle base 35 gets worn down, which also makes it easier for leaks to arise at the joint portion during the pipetting operations.
For these reasons, it is not sufficient for the pipetting apparatuses to provide with only the function that can perform the leak detection before the pipetting operations as described above, and it is desired to have another function that can detect the presence of such leaks during the pipetting operations.
In view of this problem, a leak detection method so called as a "short sample" detection method was proposed in Japanese Laid-Open Patent Application No. Hei-2-196963. Namely, in that detection method, a "Normal Sample" or "Short Sample" determination is made based on the integration value (i.e., the value corresponding to the area of a graph which shows the changes in pressure in the air hose with respect to elapsed time) calculated by integrating the pressure values of the pressure in the air hose after the aspiration of a fluid sample has been started.
However, this "short sample" detection method is only capable of detecting large leaks such as those that cause half of the prescribed volume of sample to leak out; namely, a large leak that causes the fluid sample to continuously drip out of the disposable nozzle and back into the test tube holding the fluid sample. Therefore, this method is unable to detect small leaks where sample fluid gradually flows out of the lower end opening of the disposable tip to form a drop after a proper volume of sample has been aspirated.
Another solution has been proposed in Japanese Laid-Open Patent Application No. Hei-5-249124, which discloses a pipetting apparatus and a leak detection method so called as "bubble detection method". This method is developed in view of the fact that when the volume of sample that has been aspirated into the disposable tip is insufficient due to the presence of a leak in the piping system, air is also discharged when the aspirated sample is dispensed. On the basis of this fact, in this method, insufficiency of the dispensed sample is detected by detecting such discharge of air. In more details, according to this method, just before the dispensing of the sample has been completed (i.e., after the pump has been shut off), the pressure inside the nozzle is compared with a predetermined value, and then a determination of whether or not there is any leak in the piping system is made based on whether or not the pressure inside the nozzle exceeds the predetermined value.
However, in such bubble detection method, the aspirated sample has already been dispensed into containers at the time when it is detected that there is a leak in the piping system. This means that it is not possible to immediately redo the pipetting operation when the presence of the leak is detected, because the insufficient volume of sample has already been dispensed into containers. Further, the containers which receive the insufficient volume of sample are rendered useless due to improper volume of sample. Moreover, such bubble detection method is unable to determine the presence of such a small degree of a leak that does not arise any discharge of air when the fluid sample is dispensed.